1. Field of the Invention
This invention relates to the introduction of additional elastomer latex into a latex coagulum composite.
2. Description of the Related Art
Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Carbon black, for example, is widely used as a reinforcing agent in natural rubber and other elastomers. It is common to produce a masterbatch, that is, a premixture of filler, elastomer and various optional additives, such as extender oil. Carbon black masterbatch is prepared with different grades of commercially available carbon black which vary both in surface area per unit weight and in structure, which describes the size and complexity of aggregates of carbon black formed by the fusion of primary carbon black particles to one another. Numerous products of commercial significance are formed of such elastomeric compositions of carbon black particulate filler dispersed in natural rubber. Such products include, for example, vehicle tires wherein different elastomeric compositions may be used for the tread portion, sidewalls, wire skim and carcass. Other products include, for example, engine mount bushings, conveyor belts, windshield wipers and the like.
There are a variety of methods for producing carbon black masterbatch. In one method, disclosed in U.S. Pat. No. 6,841,606 (“the '606 patent”), a carbon black slurry and an elastomer latex are combined in a vat and then coagulated by the addition of a coagulant, such as an acid. In a variation of this process, disclosed in Japanese Patent Publication No. 2005220187, natural rubber latex is diluted to 20% rubber content (from about 24% rubber) and combined with a protease to cleave amide bonds the non-rubber components of the latex. The cleavage is believed to improve the performance of the final rubber product. In another method, disclosed in U.S. Pat. No. 6,048,923, the contents of which are incorporated by reference herein, a continuous flow of a first fluid including an elastomer latex is fed to the mixing zone of a coagulum reactor. A continuous flow of a second fluid including a carbon black slurry is fed under pressure to the mixing zone to form a mixture with the elastomer latex. The mixing of the two fluids is sufficiently energetic to substantially completely coagulate the elastomer latex with the carbon black prior to a discharge end of the coagulum reactor. As disclosed in U.S. Pat. No. 6,075,084, additional elastomer may be added to the material that emerges from the discharge end of the coagulum reactor. As disclosed in U.S. Pat. No. 6,929,783, the coagulum may then be fed to a dewatering extruder.
At high loadings of carbon black, the coagulum emerges from the coagulum reactor not as a continuous flow of carbon black-elastomer composite but as a plurality of discrete carbon black-elastomer composite regions carried by a substantially coagulum-free aqueous phase. Generally, such a discontinuous material does not pass as easily through the dewatering extruder and can backflow within the dewatering extruder, causing clogging. It is therefore desirable to prepare a continuous flow of coagulum containing a high volume fraction of carbon black that can be more easily handled in an apparatus such as a dewatering extruder.